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Yogurt Connoisseur Wiki
'What is yogurt?' Yogurt is a milk product that is produced through bacterial fermentation, primarily with the species Streptococcus thermophilus and [https://en.wikipedia.org/wiki/Lactobacillus_delbrueckii_subsp._bulgaricus Lactobacillus delbrueckii subsp bulgaricus]. Lactose naturally present in milk is fermented by the bacteria to produce lactic acid. Continual production of lactic acid gradually lowers the pH of the milk culture, causing the formation of continuous acidic gel-like structures that gives yogurt its smooth consistency and sour taste. The name, yogurt, itself is a broad classification of several milk products that share the same preliminary step of bacterial fermentation. Due to its adaptability, the yogurt recipe has been heavily modified throughout history by various cultural societies to suit their culinary and nutritional needs. After fermentation, several techniques are used to modify the fermented milk product for different culinary applications, such as frozen yogurt, raita and tzatziki. 'Significance of yogurt fermentation' Yogurt has been a staple food of human societies worldwide for thousands of years, and the global yogurt market is expected to exceed sixty-seven billion dollars by 2015.Dairy Products - A Global Strategic Business Report (2012). Global Industry Analysts Inc. Yogurt is rich in protein, calcium, riboflavin, folic acid , vitamin B6 and vitamin B12 Yale-New Haven Nutrition Advisor (March-10-05) Understanding yogurt. online http://web.archive.org/web/20080529005611/http://www.ynhh.com/online/nutrition/advisor/yogurt.html. and has a number of therapeutic applications such as protection against diarrhea, reducing lactose intolerance, prevention of urogenital infection, and prevention of osteoporosis. Lourens-Hattingh, A., and Viljoen, B. C. (2001) Yogurt as probiotic carrier food. International Dairy Journal 11, 1–17 Furthermore, certain types of yogurt are claimed to have several positive effects on overall health as investigated through a variety of research methodologies. It is important to understand the fundamental biochemical processes involved in yogurt production to optimize industrial processing and ensure consumer safety. Finally, the mechanisms involved in yogurt production are an excellent example of fermentation, and can facilitate one’s understanding of energy metabolism in lactic acid bacteria. 'How is it made?' 'Standardization' Fresh milk is first standardized to adjust the desired level of fat and protein content.Lee, W. J., Lucey, J. A. (2010) Formation and Physical Properties of Yogurt. J. Anim. Sci. 23(9), 1127 – 1136. This can be done by the addition of milk powders, whey protein concentrate, or milk protein concentrate. Stabilizers can also be added to the milk to improve the texture and appearance of the yogurt. 'Homogenization' As fat in unprocessed milk normally settles at the surface, milk needs to be homogenized. Homogenization is the name given to any process that is used to mix two non-miscible solutions until evenly distributed. This is usually done by emulsifying fat into extremely small particles that can be dispersed within the aqueous component of milk. Several methods exist to homogenize milk, but one of the most common is to force unprocessed milk through small holes at high pressure, thus breaking fat globules into smaller particles and dispersing them within the solution.Trout, G. (1950) Homogenized milk : a review and guide. online http://library.wur.nl/WebQuery/clc/176536 (Accessed October 17, 2013). 'Pasteurization' Homogenized milk is then pasteurized, which involves heating foods for a short period of time. One method, called the high temperature short time (HTST) method, involves heating milk to 72°C for 15 seconds before cooling to 4°C. HTST has been shown to kill 99.999% of bacteria found within milk. Dairy Products - A Global Strategic Business Report (2012). Global Industry Analysts Inc. Another method, ultra-high temperature (UHT) pasteurization further increases shelf-life by destroying all bacterial spores. However, UHT-treated milk leads to protein denaturation, contributing to an altered taste and a change in the viscosity of milk. Overall, pasteurization significantly increases the shelf-life of milk and is vital for the production of yogurt, as it prevents biological contamination. 'Addition of Bacterial Culture' The next step involves inoculating milk with a bacterial culture and incubating at around 42°C until the product is firm. This change in viscosity occurs due to lactate production during anaerobic fermentation, which affects the structure and interaction of casein micelles in the milk (see “acidification” section below). The firmness and viscosity of yogurt is only one characteristic that plays into yogurt’s entire sensory evaluation, which also includes taste, acidity, smell, and appearance. Importance of bacterial metabolism to yogurt culture Both S. thermophilus and L. bulgaricus are facultative anaerobes, which have the ability to oxidize glucose using mole cular oxygen if it is available.Zourari, A., Accolas, J. P., and Desmazeaud, M. J. (1992) Metabolism and biochemical characteristics of yogurt bacteria. A review. Le Lait 72, 1–34 However, the conditions under which yogurt is made are generally anaerobic, requiring that both species shift their metabolism towards fermentation. In fact, both species grow best in anaerobic conditions, in part due to their inability to handle reactive oxygen species generated by aerobic metabolism. Condon, S. (1987) Responses of lactic acid bacteria to oxygen. FEMS Microbiology Letters 46, 269–280 Higuchi, M., Yamamoto, Y., and Kamio, Y. (2000) Molecular biology of oxygen tolerance in lactic acid bacteria: Functions of NADH oxidases and Dpr in oxidative stress. Journal of Bioscience and Bioengineering 90, 484–493 The role of starter cultures in yogurt production is primarily to acidify milk by converting lactose into lactate.Lee, W. J., and Lucey, J. A. (2010) Formation and physical properties of yogurt. Asian-Aust. J. Anim. Sci 23, 1127–1136''' The disaccharide lactose can be degraded to glucose and galactose, with the latter sugar entering glycolysis via the Leloir pathway (Figure 1A and B). In the Leloir pathway, a uridylyl pyrophosphate group is transferred to galactose from a molecule of UDP-glucose. The resulting UDP-galactose is epimerised to UDP-glucose and converted to glucose-1-phosphate, which rapidly equilibrates with glucose-6-phosphate (G6P) via catalysis by phosphoglucomutase. G6P is a product of the first step of glycolysis, and can therefore enter at the second step. Carbon shuttled into glycolysis ultimately finds itself in the form of pyruvate. The organism uses the free energy difference between glucose and pyruvate to synthesize ATP and reduce NAD+ to NADH (Figure 1C). In the absence of molecular oxygen as a final electron acceptor, the electron transport chain is unable to reoxidize NADH to NAD+, upon which glycolysis is dependent. To ensure the regeneration of NAD+ and the continual production of ATP from glycolysis, pyruvate is reduced to lactate using NADH as an electron donor (Figure 1D). As lactate builds, it diffuses outside of the organism. To maintain electroneutrality, protons diffuse out in equal proportions, thus acidifying the milk (Figure 1E). Acidification Yogurt develops a gel-like consistency when it is acidified by bacteria. This is due to the presence of isoelectric (pH 4.6) casein in its milk component. Casein is a protein that makes up 80% of proteins found in cow milk.Kunz, C; Lonnerdal, B (1990)."Human-milk proteins: analysis of casein and casein subunits...".American Journal of Clinical Nutrition (The American Society for Clinical Nutrition) 51 (1): 37–46.PMID 1688683. Retrieved 14 January 2011. It is present in milk as a suspension of proteins and molecules called casein micelles, which is held together by colloidal calcium phosphate (CCP) molecules and hydrophobic interactions. The internal structure of casein micelles is disrupted by the solubilization of CCP in the acidic environment resulting from the production of lactic acid. The acidic environment also reduces the negative charge on the casein molecules, which decreases electrostatic repulsion and increases hydrophobic interactions between micelles. This, in turn, reduces the steric stability of the casein micelles, which allows the creation of clusters and aggregations of casein and thus the formation of a gel-matrix structure. Synergism between microorganisms S. thermophilus and L. delbrueckii subsp bulgaricus work synergistically to stimulate growth and acid production through a phenomenon known as proto-cooperation.Zourari, A., Accolas, J. P., and Desmazeaud, M. J. (1992) Metabolism and biochemical characteristics of yogurt bacteria. A review. Le Lait 72, 1–34 Their combined efforts result in higher acidification rates, a lower final pH, increased bacterial populations, promoted aromatic compound production, and increased stability of final product. These two strains are able to work together by exchanging growth stimulants, such as folic acid and carbon dioxide, to compensate for the absence of various nutrients. In 2009, Herve-Jimenez et al. conducted a study to investigate the biochemical interactions between the two bacteria through post-genomic analyses of a co-culture with the two strains.Herve-Jimenez, L., Guillouard, I., Guedon, E., Boudebbouze, S., Hols, P., Monnet, V., Maguin, E., and Rul, F. (2009) Postgenomic analysis of streptococcus thermophilus cocultivated in milk with Lactobacillus delbrueckii subsp. bulgaricus: involvement of nitrogen, purine, and iron metabolism. Appl. Environ. Microbiol. 75, 2062–2073 Total RNA was extracted from the cultures at the 2.5 and 5.5 hour marks, and genome-wide analyses were performed on the extracted RNA using a commercial DNA microarray kit able to detect the genes present in the two strains. The pH and species-specific bacterial counts were also quantified every hour. It was discovered that when the two bacteria were cultivated together, the acidification of milk and bacteria co unts were enhanced, overall, especially between the early and late stages of growth (see Figure 2). L. delbrueckii subsp bulgaricus (■, Figure 2B) may have experienced a decrease in later stages of growth (stationary/death phase) due to its ultimate inability to produce certain amino acids. It was also found that in the coculture, there was an up-regulation of peptides, amino acid transporters, and amino acid biosynthetic pathways compared to the individual monocultures. To illustrate the symbiotic nature of the two strains, consider the up-regulation of Arg and BCAA biosynthetic pathways. Arg and BCAA are required for optimal growth in milk, particularly for protein synthesis. This biosynthetic pathway is present in S. thermophilus; however, it is not present in L. delbrueckii subsp bulgaricus. The latter bacterium benefits from being cocultured with the former by using its stores of Arg and BCAA. Surprisingly, S. thermophilus also benefits from the presence of L. delbrueckii subsp bulgaricus because depletion of its amino acid stores forces the strain to up-regulate amino acid biosynthesis in S. thermophilus, allowing for the enhanced growth of this strain. '''How do we assess the quality of yogurt? Sensory evaluation has been receiving major support from the food industry recently, as it is a cost-effective resource that can provide insight into creating a high-quality food product that can assume a large market share. Sensory evaluation provides unique information that cannot be obtained from classical approaches. Sidel, J.L., and Stone, H. (1993) The role of sensory evaluation in the food industry. Food Quality and Preference. '4,' 65--73 '' To obtain this dominant marketshare, it is imperative that these companies use feedback from their eventual consumers using sensory evaluations.Horiuchi, H., Inoue, N., Liu, E., Fukui, M., Sasaki, Y., and Sasaki, T. (2009) A method for manufacturing superior set yogurt under reduced oxygen conditions.'' 4112--4121 From a technical standpoint, researchers can only perform sensory evaluations, as there are no objective standards for testing the physical properties of the yogurt. For example, in a 2009 study performed by Horiuchi et al., the researchers were attempting to study the effect of a LT-ROF (Low Temperature, Reduced dissolved Oxygen Fermentation) production on the physical and sensory properties, such as taste and consistency of the yogurt. Ultimately, the researchers were testing a new production method for yogurt, and if the yogurt was not pleasing to the general consumer's palate, the product would not sell, as consumer preference is mostly driven by physical properties such as flavour and consistency, and very little by chemical details such as the species of bacteria used or the exact pH of the yogurt.Ott, A., Hugi, A., Baumgartner, M., and Chaintreau, A. (2000) Sensory Investigation of Yogurt Flavor Perception: Mutual Influence of Volatiles and Acidity''. J. Agric. Food Chem. ''48,' 441--450 The commercial failure of the yogurt would mean that the the experimental production method is a failure regardless of the experimental data. The fact that the researchers used a large pool of consumers who regularly consume yogurt is a major positive, as the sensory evaluation from these individuals is from a panel of experts, which eliminates many potential biases. Thus, even though a sensory evaluation is subjective, it is a crucial form of evaluation used to predict the performance of a food product on the market. 'Health Benefits of Probiotic Yogurt' For health-conscious consumers, probiotic yogurt has been shown to have a stimulatory effect on cellular immunity, reduce cholesterol levels, and promote intestinal tract health.Lourens-Hattingh, A., and Viljoen, B. C. (2001) Yogurt as probiotic carrier food. International Dairy Journal 11, 1–17 Probiotic yogurt uses microorganisms such as ''Lactobacillus reuteri RC-14 and Lactobacillus rhamnosus GR-1 in the fermentation process. These probiotic bacteria play a role in promoting healthy flora growth by persisting in the intestinal tract and synthesizing organic acids that lower pH, thereby creating an intestinal environment less desirable for harmful bacteria, such as Salmonella spp. or various strains of E. coli. In particular, administration of probiotic bacteria has been shown to alleviate symptoms of lactose intolerance due to the auto-digesting properties of probiotic yogurt. In 2001, a study conducted by de Vrese et al. has shown that the autodigesting ability of probiotic yogurt can be partially attributed to the presence of β-galactosidase, a hydrolase enzyme that hydrolyzes the β-glycosidic bond present in lactose.Vrese, M. de, Stegelmann, A., Richter, B., Fenselau, S., Laue, C., and Schrezenmeir, J. (2001) Probiotics—compensation for lactase insufficiency. Am J Clin Nutr 73, 421s–429s Consumption of probiotic yogurt in lactose maldigesters has exhibited reduced diarrhea and flatulence. This auto-digesting feature of probiotic yogurt allows the consumption of yogurt in lactase-deficient individuals. 'References' test